1. Field of the Invention
The present invention relates to a double super twisted nematic (DSTN) liquid crystal display (LCD), and more particularly, to a DSTN LCD capable of compensating for variations in the refractive index anisotropy of liquid crystals contained in two liquid crystal cells, caused by a difference in temperature, by controlling a pre-tilt angle with orientation layers.
2. Description of the Related Art
A twisted nematic (TN) liquid crystal cell having liquid crystal molecules twisted at 90.degree. angles, is widely used in small displays, such as a wrist watch or a small measuring device, due to its low voltage, low power consumption, and long life span. On the other hand, a liquid crystal cell having a narrow range of a threshold voltage and a large twisted angle is required for increasing display area and picture of quality. In a super twisted nematic (STN) liquid crystal, the twisted angle of the liquid crystals is large, somewhere in the range of 180.degree..about.270.degree., the image is displayed in yellow, green or blue, instead of black and white, according to the application an electric field. As shown in FIG. 1, a DSTN LCD adopts a liquid crystal compensation cell 20 connected to a liquid crystal display cell 10, in order to achieve optical compensation, such that an image is displayed in black and white by removing the tinted color. Polarization plates 11 and 21 are attached to exterior sides of the liquid crystal display cell 10 and the liquid crystal compensation cell 20.
The general requirement for the liquid crystal compensation cell is as follows.
The twisted angle -.phi. of liquid crystals of a second liquid crystal layer 22 of the liquid crystal compensation cell 20 is the same as the twisted angle .phi. of liquid crystals injected into a first liquid crystal layer 12 of the liquid crystal display cell 10, except for its direction which is opposite to the twisted angle .phi.. Also, assuming that the refractive index anisotropy of the liquid crystals contained in the respective liquid crystal layers 12 and 22 is .DELTA.n and the cell gaps are D1 and D2, respectively, the following relationship must be satisfied. EQU .DELTA.n.multidot.D1=.DELTA.n.multidot.D2 (1)
Here, the refractive index anisotropy represents the difference between an extraordinary refractive index and a normal refractive index.
Also, orientation layers 13, 14, 23 and 24 are processed by rubbing, such that the orientation of liquid crystals are perpendicular to each other at a contact portion between the liquid crystal display cell 10 and the liquid crystal compensation cell 20.
If the above conditions are satisfied, a linear polarized light emitted from a light source (not shown) via the polarization plate 11 is changed into an elliptical polarized light while passing through the liquid crystal display cell 10, due to a birefringent effect of the liquid crystals. However, the elliptical polarized light is changed back into the original linear polarized light while passing through the liquid crystal compensation cell 20, thereby eliminating the tinted color. Thus, a black and white display color is obtained.
In general, the refractive index anisotropies of the liquid crystals in the first and the second liquid crystal layers 12 and 22 vary with temperature. In the case of a backlight-type display, the temperature of liquid crystal of the first liquid crystal layer 12, which is near the backlight (not shown), is higher than that of the second liquid crystal layer 22 during operation. Accordingly, the amount of change in the refractive index anisotropy of the first liquid crystal is different from that of the second liquid crystal, thereby making the predetermined setting of the directions between the first and second liquid crystals, as aforementioned, ineffective.
Thus, the equation (1) is satisfied in the prior art by differing the cell gaps D1 and D2, and the type of liquid crystals implanted into the liquid crystal layers 12 and 22. Here, if the values of .DELTA.n.multidot.D1 and .DELTA.n.multidot.D2 are out of a predetermined range, leakage of light in a non-selected state, where electricity is not applied, is increased, sharply lowering contrast. Thus, in general, the cell gaps D1 and D2 are precisely controlled within a range of .+-.0.05 .mu.m.
However, in order to differ the cell gaps D1 and D2, spacers having different sizes are necessary in the manufacturing process. Also, when injecting liquid crystals, the injection pressure must be considered, and sealing after injection of liquid crystals is also complicated. In addition, the types of liquid crystals that have been developed up until now are not sufficient for compensating for the change in the refractive index anisotropy.